Superheterodyne receiver comprising automatic frequency control



F. A. DE GRooT 2,483,314 SUPERHETERODYNE RECEIVER COMPRISING AUTOMATIC FREQUENCY CONTROL Filed April 12. 1946 Sept. 27, 1949.

waif/5121455222115 62001" I INVENTOR.

ATTORNZ Patented Sept. 27, 1949 SUP-ERHETERQ'DYNE RECEIVER 1 COMPRIS IN G AUTOMATIC FREQUENCY CONTROL.

Folkert Albert de Groot, Eindhoven, Netherlands, assignor,v by. mesne...assignments, to Hartford. N ational' Bank. and Trust Company, Hartford;

Conn, as trustee Application.AprilF12, 1946, Serial No. 661,522 Inthe- Netherlands April- 12, 1943.

Sectionl, PublieLaw 690, August-8,-= 1946' Patent expires April '12, 1963 3 Claims. (01. 1791'71).

The invention relates tol improvements in. or

modifications-1 ot: the invention. described and:

claimed in". the, specification of. copending: U.. S; patent application; Serial. No. 12,1946;

The said: specification; relates to. a. super heterodyne receiver whose intermediate frequencypartcomprisesa sharpfilter for the:selec- 51011201 the carrier wave; in; which .a control voltagefonautomatic frequency-control is generated with. the aid ofa frequency discriminator of.

which'the said' sharp'filter forms part and to which; one; or. more, controlerectifiers are. con-.- nected; whilst the intermediate frequency carrier" wave is. kept. within. the. frequency-range selected: bythe: filter. solely. by controlling by means of the; said control voltage. the tuning of the; sharp filter. The; filter may comprise one:v

or; more. resonant. circuits with. reduced. damp;- ing in which. the.;damping reduction is generally eiiectedr. by means: of aback-coupled discharge:

systempr; by meansof a dischargesystem. in= which-:a-negative resistance occurs between two electrodes herein; 13031110111618. the: accentuation. of. the. carrier wave; with'respect to; the .sideband frequenoiathis: filter; is. influenced, in such a. manner by automatic;frequency-control thatthe intermedi-.

ate; frequency carrier. wave is. kept. within. the.

frequencyrrange'selectedzby the filteiz. In order to enable: variationof. the tuning. of the. filter.

throughra comparatively widerange, useis. pref.-

erablyvmade of: a filter comprising. one or: more.

resonant circuits withregenerative. feedback; The; objectcf the present invention is. to.-provid'e; an, especially suitable form of. construction of; the receiver above described which: ensuresanrin proved .1 performance.

According to the invention, the-said frequency discriminatorisccnstructedvin such manner. that'- the ;automatic;frequency-control occurs: solely or substantially. solely as; a. function of" the detuning Ofr the filter' with respect toi the carrier wave;

The: said; frequency discriminator is; usually". constructed. in; such-v manner that:- the carrien 55 661,523 ,1 filed April.

The term. selecting. is. construed;

wave applied. to the: discriminator is converted into two voltages, which: are applied respectively.

in phase andin antiphasezytm a: couple, of :pushpull connected COHtIYOIEI'GCtifiEIS and which, if.-

the tuning of thefilteris; correct, have. a given phase displacement which will be referred tohereinafter as: the normal phase. displacementand which is'usuallyz909. Now, if. the discriminator network in additionatothe filter whose tuning'is controlled comprises-zone; orrmore uncontrolled resonant circuits", the-phase displacement,

between the two said; voltages may diverge from the normal phase displacement; the filter being.

nevertheless tunedcorrectly, or beequal to: the normal phase displacement, thefilter; being tuned incorrectly, this being due to the. phase displacement brought about. by the uncontrolled circuits. It is obviousthat due to this themeeration of the. frequency-control is disturbed.

This disadvantage is, however, avoided-if care is taken. that the uncontrolled circuits. bring about the; same phase displacement of the two said voltages, or in other words if these circuits donot cause a relative-phase displacement between the said t-wo'voltagesr Care is therefore taken that the-uncontrolled.

intermediate frequency circuits of. the. network have suchaqua'lity for the frequency of the intermediate frequency; carrier wave that they do not cause .a relativephase. displacement between. the voltages applied to the controlnrectifiersing phase and in antiphase-respectively; Ifthis cone dition is satisfied, thesecircuits nolonger infiuence the value. of the control voltage-so thatv the automatic frequency-control merely depends upon the'detuning of the filter-with::respect-to the carrier wave.

In order that the inventionemay; be. clearly understood and. readily carried; into effect, it will now be set out more'fully; with reference to the accompanying drawing, in which one embodimentis:illustrated;

The single-figureof the drawing represents the, intermediate frequency portion: of a: receiver towhich the invention is applied. Anintermediate frequency amplifying tube 1- is coupled; to" the control grid circuit of a discharge tube. l via a: resonant circuit. 2 tuned ,torthezzintermediate ire-- quencyand a filter 3 formed by; a resonant; cir-- cuit with regenerative-feedback. The: filter" 3 .is

constituted by a condenser 5 and two coils-d and; 1.. connected in: series. The. damping. circuit 3 .isqcontrolled by connectinathe: latter'in threepointzconnectionzto theadischargertuber;4:.. As relatedzin-z copendingu Si; Application Serial No 661,523, it i generally necessary for the circuit to have its damping reduced to a marked extent, and the discharge tube is preferably also negatively backcoupled in such manner that the circuit is not damped by the negative feedback per se. This measure ensures that in the case the damping of the filter is reduced to a large degree, self-oscillation cannot occur under certain conditions. In the present case the negative feedback is obtained by means of a resistance 8, not shunted so far as intermediate-frequency currents are concerned, included in the cathode circuit of the tube 4. The presence of this resistance does not damp circuit 3.

The anode circuit of the discharge tube t includes a circuit 9 tuned to the intermediate-frequency and coupled capacitatively via a condenser lll to a diode-detector 1 whose audio-frequency output voltage is supplied to an audiofrequency amplifier (not shown).

In addition, the circuit 2 has connected to it a circuit l2 which is also tuned to the intermediate-frequency and which has connected to it two push-pull connected control rectifiers (diodes) l3 and M. The centre of the circuit l2 and the centre of the output resistance it of the two diodes l3 and M are connected to a coupling coil [6 which is coupled inductively to the intermediate-frequency circuit 9. The circuits 1, 9 and i2 and the coupling coil H5 constitute a discriminator network which jointly with the control rectifiers l3 and 14 connected to it constitutes a device for generating a control voltage for automatic frequency-control, said control voltage being set up across the said output resistance l5.

The control voltage is supplied to the control-grid of a discharge tube ll shown in the drawing as a triode, whose anode current is passed through a magnetising coil 8. The inductance 6 is provided with a core of high-frequency iron on which the magnetising coil I8 is fitted. Variation of the direct current passing through this coil permits of varying the inductance of the coil 6 and hence the tuning of the sharp circuit 3 within certain limits.

Owing to the presence of a sharp circuit 3 the sideband frequencies of the intermediate-frequency signal are strongly attenuated with respect to the carrier wave, so that the risk of an apparent over-modulation of the signal occurring in the event of selective fading, which would be accompanied by a Strong non-linear distortion of the low-frequency signal, is avoided. The linear distortion brought about by the attenuation of the sideband frequencies is elminated by a suitable choice of the frequency characteristic curve of the audio-frequency amplifier.

The desired operation, however, requires that the tuning frequency of the circuit 3 always corresponds within narrow limits with the frequency of the intermediate-frequency carrier wave. This conformity is ensured by the automatic frequency-control, since, in the event of a divergence occurring between the said frequencies a control 6 voltage of proper polarity is generated across the resistance and modifies the inductance of the coil 6, to such an extent that the tuning frequency of the circuit 3 is again substantially put into agreement with the carrier wave frequency.

In order to achieve such conformity, it is, however, necessary, as set out above, that the frequency-control should depend solely or substantially solely upon the detuning of the circuit 3 with respect to the carrier wave and should not depend at the same time upon the frequency difference between the carrier wave and the tuning of the uncontrolled intermediate-frequency circuits 9 and I2. According to the invention, the circuits 9 and [2 are constructed in such manner 7 that their quality factor is identical or substantially identical for the frequency of the intermediate-frequency carrier wave. If indeed the two circuits are of equal quality, they cause no relative phase displacement between the voltages supplied to the control rectifiers in phase and in antiphase respectively and consequently the output resistance I5 has occurring across it a control voltage which depends solely upon the phase displacement caused by the detuning of the circuit 3 with respect to the carrier wave, since the phase displacements occurring by reason of the frequency difference between the carrier wave and the tuning of the circuits 9 and 12, are identical and hence do not contribute towards a relative phase displacement between the said voltages.

What I claim is:

1. An amplifier circuit arrangement, particularly for use in a receiver for receiving signal voltages of carrier and sideband frequencies, comprising an amplifier having an input circuit and an output circuit for translating a first signal voltage having frequency components corresponding to said carrier and said sideband frequencies, a filter circuit sharply tuned to a frequency corresponding to said carrier frequency and coupled to the input circuit of said amplifier to apply a signal voltage of frequency corresponding to said carrier and sideband frequencies to said amplifier, means to couple said filter circuit to said amplifier to derive and apply to the input circuit of said amplifier a second signal voltage of frequency corresponding to said carrier frequency to reduce the damping of said filter, a frequency discriminator circuit having a first resonant circuit coupled to the input circuit and a second resonant circuit coupled to the output circuit of said amplifier to produce a frequency control voltage, means responsive to said control voltage to tune said filter sharply to an intermediate frequency corresponding to said carrier frequency, said first and said second resonant circuits having quality factors such that said frequency control voltage varies substantially solely proportional to the difference in frequency between the frequency to which said filter is tuned and the frequency of said signal voltages, and means coupled to the output circuit of said amplifier to derive an output voltage comprising said first signal voltage and said second signal voltage and having carrier frequency components relative to sideband frequency components greater than the carrier frequency component of said first signal voltage.

2. An amplifier circuit arrangement, particularly for use in a receiver for receiving signal voltages of carrier and sideband frequencies, comprising an amplifier having an input circuit and an output circuit for translating a first signal voltage having frequency components corresponding to said carrier and said sideband frequencies, a filter circuit sharply tuned to a frequency corresponding to said carrier frequency and coupled to the input circuit of said amplifier to apply a signal voltage of frequency corresponding to said carrier and sideband frequencies to said amplifier, means to couple said filter circuit to said amplifier to derive and apply to the input circuit of said amplifier a second signal voltage of frequency corresponding to said carrier frequency to reduce the damping of said filter, a frequency discriminator circuit having a first resonant circuit coupled to the input circuit and a second resonant circuit coupled to the output circuit of said amplifier to produce a frequency control voltage, means responsive to said control voltage to tune said filter sharply to an intermediate frequency corresponding to said carrier frequency, said first and said second resonant circuits having substantially equal quality factors whereby said frequency control voltage varies substantially solely proportional to the difference in frequency between the frequency to which said filter is tuned and the frequency of said signal voltages, and means coupled to the output circuit of said amplifier to derive an output voltage comprising said first signal voltage and said second signal voltage and having carrier frequency components relative to sideband frequency components greater than the carrier frequency component of said first signal voltage.

3. An amplifier circuit arrangement, particularly for use in a receiver for receiving signal voltages of carrier and sideband frequencies, comprising an amplifier having an input circuit and an output circuit for translating a first signal voltage having frequency components corresponding to said carrier and said sideband frequencies, a filter circuit sharply tuned to a frequency corresponding to said carrier frequency and coupled to the input circuit of said amplifier to apply a signal voltage of frequency corresponding to said carrier and sideband frequencies to said amplifier, means to couple said filter circuit to said amplifier to derive and apply to the input circuit of said amplifier a second signal voltage of frequency corresponding said carrier frequency to reduce the damping of said filter, a frequency discriminator circuit having a first resonant circuit coupled to the input circuit and a second resonant circuit coupled to the output circuit of said amplifier to produce a frequency control voltage, means responsive to said control voltage to tune said filter sharply to an intermediate frequency corresponding to said carrier frequency, said first, said second resonant circuits having quality factors producing substantially equal phase displacements of the voltages there across whereby said frequency control voltage varies only in proportion to the frequency difference between the frequency to which said filter is tuned and the carrier frequency of said received signal voltages, and means coupled to the output circuit of said amplifier to derive an output voltage comprising said first signal voltage and said second signal voltage and having carrier frequency components relative to sideband frequency components greater than the carrier frequency component of said first signal voltage.

FOLKERT ALBERT DE GROOT.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,642,173 Round Sept. 13, 1927 ,681,532 Gardner Aug. 21, 1928 2,121,103 Seeley June 21, 1938 2,302,893 Roberts Nov. 24, 1942 Certificate of Correction Patent No. 2,483,314 September 27, 1949 FOLKERT ALBERT 1m GROOT It is hereby certified that errors appear in the printed specification of the above numbered patent requiring correction as follows:

Column 6, line 4, after the: word corresponding insert to; line 37, list of references cited, after 2,121,103 Seeley June 21 1938 insert 2,200,038 Mountjoy et al May 7', 1.940;

and that the said Letters Patent should be read with these corrections therein that the same may conform to the record of the case in the Patent Oflice.

Signed and sealed this 28th day of February, A. D. 1950.

THOMAS F. MURPHY,

Assistant Commissioner of Patents. 

